Direct injection gasoline engine technology is becoming increasingly popular due to the potential to reduce fuel consumption with little compromise to what the automotive gasoline engine offers today in terms of power density and cost. In order to provide a better understanding of the invention described below, some background as to different types of fuel injection and combustion systems is appropriate.
Direct injected (DI) engines operate with fuel being directly Injected into the engine cylinders (ie combustion chambers), as compared to conventional fuel injected (FI) engines (also called manifold injected or PFI engines), where the fuel is injected into the intake manifold immediately behind the Inlet valve of the combustion chamber in order to carburise the fuel.
“High pressure direct injection”(HPDI) fuel systems are typically those where the injected fluid is solely fuel, and injection is carried out at high pressure (typically 50 to 120 bar). These systems have to be contrasted with so called “air (or gas) assisted direct injection” (AADI) fuel systems, where fuel is metered into a mixing chamber, of a delivery injector typically at constant pressure for mixing with air. This “dual fluid” is then injected at low pressure (typically 6 to 8 bar) into the cylinder combustion chamber.
Typically, with DI engines, the fuel injection or delivery device penetrates the combustion chamber through the cylinder head. An engine having an injection device that penetrates into the combustion chamber through a cylinder head with a fuel delivery direction having a generally axial orientation with respect to the combustion chamber axis may be referred to as a central direct injection engine. Engines that have an injection device that penetrates into the side of combustion chamber In order to deliver the fuel stream in a generally radial direction are commonly referred to as side direct injection engines.
The location of the fuel injector outlet in the combustion chamber in relation to the fuel ignition source (e.g. spark plug) and the type of fuel injection system employed, HPDI or AADI, influences the choice of specific charge transport mechanisms to ensure proper mixing of fuel with combustion air supplied through the inlet port, and ignition of the fuel-air mixture within the combustion chamber.
FIG. 1 of the accompanying drawings illustrates, in a highly simplified and schematic manner, three different mechanisms of charge (i.e. either fuel or fuel-air mixture) guidance within a cylinder of a four stroke internal combustion gasoline engine 5, the cylinder being identified at 10 (cylinder head and cylinder body). A reciprocating piston at 15 delimits the internal combustion chamber 20. For clarity purposes, the inlet and outlet valves have been omitted from FIG. 1, but their relative arrangement within cylinder head 10 and their location as viewed from the combustion chamber 20 is well known and illustrated in FIG. 6 for a four valve cylinder head type. It will be noted that the spark plug 30 is arranged with the spark gap close to the central axis of the cylinder 10. A direct fuel injector is represented at 25 at different locations, Le in central or side injection engine arrangement.
In “spray guided” (direct injection) combustion systems, the injector is typically located and arranged to direct the fuel spray to the spark plug gap so that reliance on secondary mechanisms for transporting fuel to the spark plug gap is minimised. In FIG. 1, this system arrangement is of a central spray guided type. On the other hand, so called “wall guided” combustion systems provide for transportation of the injected fuel to the plug gap by secondary mechanisms such as interaction of the injected fuel with a piston bowl and/or air motion within the cylinder. In so called “charge motion or air guided” systems, the motion of air entering into the combustion chamber through the inlet port(s) is used to achieve said transport of fuel, through swirl and/or tumble motion, towards the ignition zone.
The result of both charge guided and wall guided transportation methods is a longer preparation time of the fuel to create the carburated charge. This is typically the case for single fluid injection systems, as compared with AADI injection systems. This longer preparation time is in particular also important for HPDI systems because in such systems the injected fluid is solely fuel and there is the need to generate a fuel-air “cloud”, ie the carburated charge, within the cylinder.
Modern gasoline direct injection engines generally attempt to generate a non-uniform distribution of fuel within the combustion chamber. This non-uniform distribution is commonly referred to as a stratified charge and means that typically one-region of the combustion chamber has a greater concentration of fuel than the remainder of the combustion chamber during certain load conditions. Engines that are adapted to operate in this fashion are commonly referred to as stratified charge engines. Stratified charge engines are theoretically free from the air fuel ratio limitations of homogenous charge engines (e.g. manifold injected engines are typically homogenous charge engines) where the intention is to achieve a uniform mixture of air and fuel throughout the combustion chamber prior to ignition under all load conditions. In contrast, a typical stratified charge engine operates with a stratified charge at low speed and low load conditions and
operates with a homogenous charge at higher speed and load conditions.
In order to generate a stratified charge in an engine with a central injected fuel delivery system, the injection device would typically be timed to inject a fuel spray into the combustion chamber later in the combustion cycle compared with the injection timings required to generate a homogenous charge. By injecting later in the cycle, the fuel spray has a limited amount of time to mix with the intake air in the combustion chamber, resulting in a stratified charge of air and fuel. Homogenous charges on the other hand may be generated by injecting relatively early in the combustion cycle so that the fuel spray injected by the Injection device has sufficient time to mix with the intake air and thereby form a homogenous mixture of air and fuel (ie carburated fuel) within the combustion chamber.
As noted above, a sub-set of direct injected (DI) engines inci, a “spray guided” direct injected fuel combustion systems. In such engines, th injection device outlet is located such that the fuel spray Is issued so as to penetrate into the combustion chamber in close proximity to a fuel ignition device, typically the spark plug. A spray guided direct Injected fuel combustion system may be of a central Injection type. Accordingly, when a central injection spray guided engine generates a stratified charge, this stratified charge can generally be ignited by the ignition device as the spray from the centrally located injection device passes the ignition device. Typical timings attempt to ignite the tail of the spray so that the Injection device outlet is closed when combustion occurs. That is, spray guided systems do not transport the fuel spray from the injection device to the ignition device by use of a secondary means, as Is commonly the case with “wall guided” and “charge motion/air guided” systems. Accordingly, ignition occurs directly off the fuel spray delivered by the injection device. However, two of the major problems associated with single fluid implementation of spray guided combustion systems are spark plug durability and steep air/fuel ratio gradients which lead to poor robustness of combustion. These drawbacks are largely avoided with AADI combustion systems which has allowed their application to spray guided systems. However, even with AADI spray guided systems it has been found that the levels of emissions present in the engine out gasses indicate that spray guided systems can still generate imperfect and/or partial combustion of hydrocarbons present in the fuel. Hence there is a need, with ever more stringent emissions regulations, to reduce engine out emissions In order to avoid catalyst solutions that are either expensive or un-economic. There is also a need to improve fuel economy through improved combustion.
Stratified charge engines generally have fuel consumption benefits over homogenous charge engines. However, as a stratified charge has a lean-air fuel ratio when the combustion chamber is considered as a whole, it has generally been found that at various engine operating points the levels of oxides of nitrogen (NOx) emitted by stratified charge engines are higher than for comparable homogenous charge engines. Thus operating a spray guided direct injection fuel system in a stratified manner has the potential to further degrade the engine out emissions generally expected.
Stratified charge engines can generally operate under what is referred to as “fuel-led” control strategies where the amount of fuel delivered to the combustion chamber is independent of the quantity of air delivered to the combustion chamber by the intake manifold. This results in the engine torque and load being directly proportional to the amount of fuel delivered to the engine. In contrast, in a typical homogenous charge engine, the amount of fuel that can be delivered to the engine is dictated by throttle angle and hence air flow to the combustion chamber. Accordingly, such control strategies are referred to as “air-led” control strategies.
Typically, a “fuel-led” control strategy provides sufficient fuel to the combustion chamber such that the combustion chamber when viewed overall has a lean mixture of air and fuel. However as the fuel is localised to a specific region of the combustion chamber, this region is itself generally ignitable and so some of the issues associated with ignition and combustion of overly lean air fuel mixtures in homogenous charge engines are either reduced, preferably so as not to be significant or are eliminated altogether.
Having regard to the many variables that influence efficient combustion and the different types of fuel injection and combustion systems described above, with their distinctive advantages in some areas of engine operation/load, the system of choice is still unclear. Investigations continue on both wall or charge motion guided systems, as well as jet or spray guided systems, in relation to both HPDI and AADI systems. Recent studies such as Niefer, HG et al, “The DI Gasoline Engine: Quo Vadis-where does the road lead?”, Vienna Motor Symposium, 1999; and Fraidl, GK et al, “Gasoline Direct Injection-The Low Fuel Consumption for EURO4”, Vienna Motor Symposium, 1993 have identified the spray guided combustion system as the one with perhaps the most potential for a direct injected automotive gasoline engine. By the same token however, these last two documents have identified some areas of concern as regards emission control, see above. Reference should be made to these two documents which are hereby incorporated by way of short-hand cross-reference.
The present invention has been conceived in slight of a perceived need for further improvement of direct-injected, spark ignited internal combustion engines, in particular gasoline four stroke engines having poppet-style inlet and exhaust valves that use spray guided fuel injection systems and particularly AADI spray guided fuel injection systems. The present invention is more in particular concerned with mechanisms that influence the motion and/or containment of a stratified charge within the cylinder combustion chamber of a spray guided fuel injection system to thereby positively affect fuel consumption and emission levels in four-stroke, spark-ignited, stratified-operation engines.